The window covering with rewindable pall generally requires guiding cord to lift or lower the pall thereof. Therefore, the smoothness of the cord operation will influence the operation of those window coverings.
FIG. 1 shows a headrail hardware for hanging window coverings described in U.S. Pat. No. 4,623,012. As shown in this figure, a plurality of cords 13 are fixed on the chip 12 of a capstan 11 with one end thereof. The capstan 11 supported by a plurality of brackets 14 winds the cord 13 on the outer surface thereof in single layer when being driven by an output axis 10. The capstan 11 begins to move laterally when sufficient friction has developed between the capstan and the cord which has been wound on to it. This provides space for the cord to wind onto the capstan in a single layer. When the pall is lowered to the lowest point and the output axis 10 is rotated in the same direction, then cord 13 will be wound on the capstan 11 reversely, thus lifts again the pall.
However, there are several problems in the conventional lifting device.
First, two or more cords are required to wind/loose the pall such that the cord near the driving means has larger torque and the cord far from the driving means has smaller torque. Therefore, both ends of the pall will be wound to different height. The problem is more serious when the pall is wound to a higher position.
Second, the cross-section capstan is hard to be a perfect circle by manufacture. Therefore, the fore is hard to uniformly exert onto the capstan.
Third, the pall can be kept winding such that the cord is reversely wound when the pall is at the lowest point. However, the lowest point is hard to define especially when a machine-control is used.
Fourth, the plurality cords are arranged on the capstan by a plurality of clips, the assembling of those cords and clips on the capstan is troublesome.
It is the object of the invention to provide a lifting device for window covering, by which the cords can be uniformly wound, the assembling can be simplified and the winding operation can be stopped at the lowest point.
In an aspect of the invention, the invention device has thread shaft with stop part which axially arranged on and moves synchronously with a transmission shaft. Each of the plurality of cords is fixed on corresponding thread shaft, wherein the outer thread of the thread shaft is matched with the inner thread of the through hole of the fixing stage. The thread shafts are driven by the transmission shaft and rotated, moved forward/backward by the counter action of the thread part of the fixing stage. The thread shaft can receive the cord with the same length and move the pall simultaneously. At the same time, the cord is wound around the groove of the outer thread of the thread shaft. The thread shaft and the transmission shaft of the present invention are slidably arranged other than fixed such that they can be assembled in either order. Therefore, both ends of the pall can be ensured to have same height and the assembling and manufacturing thereof are simplified.
To achieve above object, the inventive lift device comprises a transmission shaft linked with a driving means; a plurality of thread shaft with hole slidably arranged on the transmission shaft to rotate synchronously with the transmission shaft, each thread shaft having outer thread and a stop part on one end thereof; a plurality of fixing stage on the curtain body, each fixing stage housing one thread hole of the corresponding thread shaft and having inner thread corresponding to the outer thread of the thread shaft; and a plurality of cords, each fixed on one corresponding thread shaft.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: